The word "tho" is a colloquial contraction of "though." It's commonly used in informal speech and writing to indicate contrast or to add a thought, often sounding casual or relaxed. It functions similarly to "however" or "although," usually signaling that there's more to the statement being made or implying a kind of concession.
"Tho" is derived from the Old English word "þonne," which means "then," and was later influenced by the word "though." Its usage as a casual form became popular in 20th-century American English, especially in online communication and texting.